1. Field of the Invention
The present invention relates to a material for the formation of hydrophilic contact lenses, and particularly to terpolymer hydrogels formed from a hydroxy alkyl acrylate or methacrylate together with very small amounts of a non-water soluble diester and a free acid.
2. Description of the Prior Art
Acceptable hydrophilic or "soft" contact lenses should exhibit certain desirable properties which in the past have been difficult to achieve or control. Preferably the hydrated lens should be strong yet flexible, so that if stretched, the lens will not tear, and will recover its original shape without distortion of the optical quality. The lens should have a low pH dependence, so that changes in the pH of tear fluids in the eye will not cause variation in the shape, water content or optical properties of the lens. Further, the lens material should exhibit consistent expansion upon hydration, to insure repeatability in the manufacturing process. One object of the present invention is to provide a soft contact lens material having these desirable qualities.
Another desirable feature, not heretofor possible, is the formation of soft contact lenses in which the water content is so great, typically about 60 to 80% by weight, that tear exchange with the cornea is not inhibited by the lens. Tear fluid does not stagnate under the lens. As a result, lenses of this type can be left in the eye for indefinitely long periods of time.
A known formulation for hydrophilic contact lens material is set forth by Wichterle and Lim in the U.S. Pat. Nos. 2,976,576 and 3,220,960. Three dimensional, sparingly cross-linked copolymer hydrogels are produced by polymerizing in aqueous solution a monomeric acrylic or methacrylic acid ester having a hydrophilic (e.g., hydroxy) ester moiety together with less than one mol percent of a water soluble diester formed from a polyhydroxylic alcohol having excess hydrophilic groups and an olefinic acid such as acrylic or methacrylic acid.
In the Wichterle and Lim copolymer material, properties of shape stability and swelling capacity both are altered by changing the ratio between the monomer and diester. This dual effect of the water-soluble diester concentration results since the diester plays two roles. First, it acts as the cross-linking agent, thereby establishing the physical properties of the hydrogel. Secondly, since the diester is water soluble and has excess hydrophilic groups, it increases the hydration capability of the hydrogel. However, there is no separate control of physical characteristics and hydration percentage, since a change in diester concentration alters both properties. An object of the present invention is to provide a hydrophilic polymer material in which shape stability and water content upon hydration can be controlled separately.
Another characteristic of the Wichterle and Lim material is that it is polymerized in aqueous solution. In other words, the monomer and water soluble diester constituents are premixed and reacted in a water solution. As a result, the hydrogel that is formed is already hydrated and soft. Lenses must be made by a molding process in which the material is polymerized within a lens-shaped mold. Conventional grinding techniques cannot be used. Another object of the present invention is to provide a hydrophilic material that is formed in the rigid, unhydrated state. This permits lens fabrication using conventional grinding techniques, with subsequent hydration.
Control of the physical properties of hydrophilic contact lens material also is discussed in the U.S. Pat. No. 3,503,942 to Maurice Seiderman. There, stiff but flexible hydrophilic plastic materials are produced by introducing a controlled, increased amount of cross-linking into the polymeric structure. This is accomplished by combining (a) an acid and inhibitor-free monomer of hydroxy alkyl acrylate or methacrylate, (b) a minor amount of up to about 35% by weight of alkyl acrylate or methacrylate wherein the alkyl group contains from 1 to about 20 carbon atoms, and (c) from 1% to about 50% by weight of a cross-linking monomer such as an acrylic diester. Addition of this longer chain alkyl methacrylate gives better polymer chain strength, and aids in restricting water swellability of the resultant product.
Another approach is taught in the U.S. Pat. No. 3,532,679 to Robert Steckler. There, transparent hydrogels having good shape or body retaining characteristics and good volume stability are produced by simultaneously copolymerizing and cross-linking a mixture of comonomers including N-vinyl lactam, an acrylate ester monomer and from about 0.5% to about 12% by weight of a diester cross-linking agent such as polyethylene glycol dimethacrylate.
Thus it is characteristic of the prior art to use relatively large amounts of diester cross-linking agent, together with an acid-free hydroxyalkyl monomer, to obtain hydrogel materials having physical properties acceptable for soft contact lenses. Additives such as a longer chain alkyl polymer, or a vinyl lactam than are used to control these properties. In contrast to the prior art, it has now been found that hydrophilic lenses having excellent stretch characteristics, low pH dependence and controllable expansion upon hydration advantageously are formed from compositions having a major portion of a hydroxy alkyl acrylate or methacrylate together with very small amounts of free acid (less than about 10% and preferably below about 1.5% for low pH dependence and optimum strength) and a diester cross-linking agent (preferably less than 0.25%) having no free hydrophilic groups. The principal object of this invention is to provide such improved materials and compositions. The physical properties of the resultant terpolymer are controlled by the concentration of diester, while the percentage of hydration of the material is separately controlled by the free acid concentration.